1. Field of the Invention
The present invention relates to a four-wheel vehicle having in-wheel motors in front and rear wheels such that a kingpin offset of the front and/or rear wheel is determined to improve the stability of the vehicle.
2. Description of the Related Art
In-wheel motor vehicles are known in the art in which electric motors are provided in the front and rear wheels as driving means. In such in-wheel motor vehicle, there is a disadvantage that a heavy steering effort may be felt because each wheel has an electric motor, a ball joint in a suspension system is consequentially placed inward of the vehicle, and thus the kingpin offset becomes large. In order to prevent such disadvantage, Japanese Patent Application Publication No. 5-116545 (JP-A-5-116545), for example, describes that a wheel disc is formed in an inward concave shape along an axle shaft, the ball joint of a lower arm is disposed in the space determined by the concave shape to be close to the wheel disc, and thus the kingpin offset is made small.
Japanese Patent Application Publication No. 2005-329817 (JP-A-2005-329817) describes an in-wheel motor having a hollow first motor and a hollow second motor concentric with each other. The outside diameter of the second motor is determined to be smaller than the inside diameter of the first motor so that a part of the second motor is positioned inside the first motor. Accordingly, a lower joint is disposed adjacently to an inner peripheral surface of a rim of the wheel, and a kingpin axis is determined at an appropriate position and angle.
In addition, Japanese Patent Application Publication No. 2006-67646 (JP-A-2006-67646) describes the control of driving and braking amount of an in-wheel motor based on a gyro moment about the kingpin axis and the kingpin offset of the in-wheel motor to suppress vibration caused by the gyro moment.
However, the cited references do not describe or teach at all how to determine the kingpin offset to stabilize the behavior of the vehicle.
Regarding the determination of the kingpin offset, the conventional rear-wheel-drive four-wheel vehicle with an engine as the driving means has, as shown in FIG. 4A, driving force and braking force are generated at different points in the wheel, and therefore, it is possible to determine the point of application of the driving force and the point of application of the braking force on the opposite sides across the kingpin axis. In other words, the point where the driving force is generated is the center of the wheel with a drive shaft, the point where the braking force is generated is the contact patch of a tire, and therefore, those forces are generated at different positions. The kingpin axis (or fictitious kingpin axis if the suspension system does not have the kingpin axis; hereinafter, the actual kingpin axis and the fictitious kingpin axis are collectively referred to as “kingpin axis”) is positioned between the point where the driving force is generated and the point where the braking force is generated to be set in a negative offset. Therefore, the moment of a force is generated about the kingpin axis in braking to point the tire to toe-in, and thus the vehicle can stabilize.
However, the in-wheel motor vehicle has, as shown in FIG. 4B, both the points of application of the driving force and the braking force on the contact patch of the tire, so that the kingpin axis cannot be positioned, as the conventional vehicle, between the point where the driving force is generated and the point where the braking force is generated, and therefore, the kingpin offset needs to be set to zero. Nevertheless, the behavior of the vehicle may become unstable due to the change of bound or rebound of the vehicle caused by weight shift during the braking and the driving.